All-SrTiO3 field effect devices made by anodic oxidation of epitaxialsemiconducting thin filmsE. Bellingeri, L. Pellegrino, D. Marré, I. Pallecchi, and A. S. Siri Citation: J. Appl. Phys. 94, 5976 (2003); doi: 10.1063/1.1613373 View online: http://dx.doi.org/10.1063/1.1613373 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v94/i9 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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JOURNAL OF APPLIED PHYSICS VOLUME 94, NUMBER 9 1 NOVEMBER 2003

All-SrTiO 3 field effect devices made by anodic oxidation of epitaxialsemiconducting thin films

E. Bellingeri,a) L. Pellegrino, D. Marre, I. Pallecchi, and A. S. SiriINFM–Lamia, Corso Perrone 24, 16152 Genova, Italy and Dipartimento di Fisica, Universita` di Genova,Via Dodecaneso 33, 16146 Genova, Italy

~Received 8 May 2003; accepted 5 August 2003!

We report a field effect device fully made of strontium titanate~STO!. This perovskite-type materialis very attractive for oxide electronics both for its notable dielectric properties as well as for itssemiconducting properties in the doped state. We exploit both of these properties by developing afield effect device in which oxygen deficient STO acts as a conducting channel and stoichiometricSTO as a dielectric barrier. Such a barrier is obtained by electrochemical oxidation of the surface ofan oxygen deficient semiconducting STO film, deposited by pulsed laser ablation in ultrahighvacuum conditions. The channel conductivity is varied by the application of an electric fieldbetween the channel itself and a metallic gate deposited onto the dielectric barrier. Modulationcapability of more than 60% is achieved by applying potential lower than 1 V. Conductivity changesare due to electrostatic induced variations of the charge carrier density~n!. This result is confirmedby Hall effect measurements during gate biasing. The very good agreement of the measuredn withthe value calculated from the device capacitance proves the electrostatic origin of the effectobserved. ©2003 American Institute of Physics.@DOI: 10.1063/1.1613373#

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I. INTRODUCTION

Strontium titanate is a band insulator (Egap'3.2 eV)1,2

with a perovskite crystal structure. It has long been studfor its unique dielectric properties, i.e., its very large dieletric constant, that is nearly 300 at room temperature10 000 at low temperature3–5 and nonlinear behavior withelectric field applied.6 Lately, it has been the object of widespread interest in the world of oxide electronics and itoften integrated into epitaxial heterostructures of oxidMany examples of prototypes of electronic devices basedthe exotic properties of perovskite oxides already existmany new ones can be easily devised, and exploit phenena such as superconductivity, ferroelectricity, ferromnetism, antiferromagnetism, charge and spin ordering, cosal magnetoresistance, and the metal–insulator transiArguably it can be said the strontium titanate SrTiO3 isamong the most used components in these oxheterostructures.7–15 In a few cases, oxide devices have befabricated which do not just exploit SrTiO3 dielectric prop-erties, but rather its transport properties.16,17 It becomes me-tallic with a lower doping concentration than that usuanecessary for transition metal oxides.18 In the metallic stateat low temperatures it exhibits mobilities higher th104 cm2 V21 s21 and residual resistivity ratios~RRRs! up to4000. Electron doping is obtained either by chemical subtution or by deviation from oxygen stoichiometry. The mocommonn-type substitutions are lanthanum on the strontisite19 and niobium on the titanium site,20 but different oneshave also been successfully tried such as antimony on a

a!Electronic mail: bellingeri@fisica.unige.it

5970021-8979/2003/94(9)/5976/6/$20.00

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nium site.21 Yet oxygen stoichiometry seems to be the easway to dope SrTiO3 and the most efficient in terms of carriemobility.22

The dramatic effect of oxygen stoichiometry on transpproperties of SrTiO3 is well known: exact stoichiometrycauses the compound to be insulating, while oxygen dciency brings about a metal–insulator transition even at qlow doping concentration. Systematic work on the oxygcontent and its relationship to transport performancesSrTiO3 heteroepitaxial thin films can be found in thliterature.23–25 Under these premises, the idea of realizifull-SrTiO3 field effect heterostructures is natural: semicoducting or metallic oxygen deficient SrTiO3 can be used asan active channel, while stoichiometric insulating SrTiO3 canbe used as an insulating layer for the gate voltage.

Several groups have reported the use of SrTiO3 as adielectric in metal–oxide–semiconductor field effect transtor ~MOSFET! heterostructures both with traditional Schannels26 and in full perovskite devices;27,28 nevertheless,the problem of growing epitaxial heterostructures compoof oxygen deficient SrTiO3 metallic layers together with stoichiometric SrTiO3 insulating layers is not solved yet, andis not clear whether, by exploiting the dynamic characterdeposition techniques in competitive thermodynamic equirium, any possibility of obtaining such heterostructures doexist at all. In this work we explore an alternative wayrealizing all-SrTiO3 heterostructures for field effect devicethat is, by electrochemical oxidation of the surface layer ometal oxygen deficient SrTiO3 film. This technique has al-ready proved to be efficient to modify the doping level, i.the oxygen content in ceramic perovskite samples29–31 andthin films.32 On the other hand, anodic oxidation has alreabeen employed to form gate oxides in a more controlled wthan thermal oxidation in devices realized with tradition

6 © 2003 American Institute of Physics

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5977J. Appl. Phys., Vol. 94, No. 9, 1 November 2003 Bellingeri et al.

semiconductors.33,34Electrochemical oxidation is very effective in creating insulating layers because the process istrinsically more efficient for conducting regions or defecsince it allows ‘‘repair’’ of leaky points selectively.

II. EXPERIMENT

Strontium titanate thin films are deposited by pulsedser ablation in ultrahigh vacuum~UHV! conditions(1029 mbar) from single crystal targets. Epitaxial films witypical thicknesses of 15–80 nm are deposited on LaA3

substrates at 700 °C using a laser fluence of 2 J/cm2. Highstructural quality and atomically flat surface morphologythe samples are observed by four-circle x-ray diffract~XRD! and atomic force microscopy~AFM!. X-ray reflectiv-ity, finite size effect, and spectroscopic ellipsometry are rtinely used to determine film thicknesses.

Different film patterns can be obtained by shadow maing technique: as examples, the Hall cross shape for transproperties measurements and large surface films for captance measurements.

Resistivity and Hall effect measurements are performon as-deposited films, from room temperature down to 4.2in magnetic fields up to 5 T: typically semimetallic behaviis observed for a number of carriers between 1019 and1020 e/cm3. Electrical contact on the samples is realizedultrasonic bonding with Al~Si! wires.

The electrochemical oxidation process is performed istandard two electrode cell or by realizing a microcell withdroplet of electrolyte on the film. The working electrodethe film surface whereas the counterelectrode is platingauze. In the case of the microcell the gauze is shapeorder to retain the droplet. The electrolyte used for anooxidation is 0.01–0.1 M solution of NaOH in de-ionizewater. The areas of the contacts and of the not-insulaAl ~Si! wire are protected during the oxidation process btwo component epoxy resin.

Cyclic voltammetry measurements are performed witcomputer controlled Amel potentiostat in the same elecchemical cell used for sample fabrication.

During device manufacturing either the oxidation curreis monitored while the voltage is controlled~potentiostaticmethod! or the other way around~galvanostatic method!.The electrical resistance of the channel is also measureding the oxidation process. This electrical resistance depeon the thickness of the conducting part and the value itquires in real time allows one to determine the depth reacby the oxidation process. Devices are realized on films wthickness in the 30–50 nm range leaving about 3–5 nmconducting STO, which means we stop the process wheincrease of almost one order of magnitude with respect tooriginal resistance is measured. It is likely that the interfabetween the insulating and conducting parts is not sharpthis case, this thickness value is to be considered as anfective one.

The device is then completed by deposition of the mtallic gate: a Au or Ag electrode is thermally evaporated othe oxidized insulating region of the films. The geometrythe devices, shown in Fig. 1, allows us to measure both

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variation of resistivity ~longitudinal contact! and the Halleffect ~transverse contact! as a function of the gate voltageThe dielectric properties of the oxidized barrier are charterized byI –V characteristic and capacitance measureme

For field effect measurements, a very low frequencycurrent~1–10 Hz! is fed into the channel and the corresponing ac voltage drop is measured by a lock-in amplifier, whthe gate is dc biased. Some measurements are also perfowith a dc current in the channel and by applying an ac voage to the gate and they obtain the same results.

III. RESULTS AND DISCUSSION

A. Structural, morphological, and chemical propertiesof as-grown films and oxidized layers

The SrTiO32d films grown under optimized conditionshow high structural quality. In Fig. 2 an x-ray diffractiopattern of an as-deposited film is shown. In the insetfinite size effect around the 002 STO reflection is magnifishowing evidence of the perfect uniformity of the growth.large value for thec axis of about 4.0 Å is obtained from thpattern. XRD scans around 110 reflection allowed us to msure ana lattice parameter of 3.79 Å, equal to that of thsubstrate. The large mismatch between the LAO and Slattice parameters, which are 3.789 and 3.905 Å, resptively, induces strong lattice strain in the STO thin filmwhose crystalline cells shrinking in the basal plane and elgate in thec direction, conserving the cell volume. No strarelaxation is observed up to the highest thickness depos~about 80 nm!. f scans~Fig. 3! of the film and substrate 110reflections indicate ‘‘cube on cube’’ epitaxial growth.

FIG. 1. Schematic of the Hall cross shaped devices.

FIG. 2. X-ray diffraction pattern of an as-deposited SrTiO3 thin film. In theinset finite size effect fringes around the 002 reflection indicate the peruniformity of the growth. From the fringes a period with thickness of 20 nis estimated.

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5978 J. Appl. Phys., Vol. 94, No. 9, 1 November 2003 Bellingeri et al.

The surface measured by AFM~Fig. 4! is very smoothwith roughness less than one unit cell in very large areassome regions, steps originating from twinning planes ofunderlying LAO substrate are observed.

With the aim of producing a stoichiometric SrTiO3 layeron the surface of these oxygen deficient semiconducfilms, we anodically oxidize films with a thickness in thrange 40–80 nm. To optimize the insulating layer propertwe test different procedures and study the electrochemreactions by cyclic voltametry. In Fig. 5 electrochemical cclic voltametric measurements of a SrTiO32d film in NaOH0.01 M and a Pt counterelectrode are reported; in order toas close as possible to the process used in device manturing, no reference electrode is employed. Four differregimes can be identified: up to;0.75 V no current is de-tected, then the rapid current rise is related to oxygen retion with the film:

SrTiO32d12dOH2⇔SrTiO312de21dH2O.

For voltage in the range of 1.2–1.6 V the current increaslowly, indicating a diffusion limited reaction. For voltagehigher than;1.6 V another rapid increment of the currentpresent and may be related to possible decomposition ofilm. Oxygen evolution at the anode is only observed in tregion.

Different oxidation procedures are tested, namely, gvanostatic or potentiostatic oxidation. The galvanostatic o

FIG. 4. AFM image of the surface of an as-deposited SrTiO3 thin film.

FIG. 3. f scans of the 101 reflection of~a! SrTiO3 thin film and ~b! LAOsubstrates, indicating cube on cube epitaxial growth.

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dation is carried out in such a way that the differencepotential at the film–electrolyte interface never exceedsV, meaning current density lower than;4 mA/cm2, in orderto avoid possible decomposition. This procedure causesincrease of the resistance of the sample, but formation oinsulating barrier is not observed. TheI –V characteristics ofthe oxidized layer are linear, indicating ohmic type condution even for very high resistance values. Moreover, this vtical resistance is of the same order of magnitude as thain-plane one. This result can be interpreted as almost unifdiffusion of oxygen in the entire film, without the formatioof an oxidized barrier.

During potentiostatic oxidation at low voltage~,1.5 V!the current decreases in time and the process stopsabout ten minutes. This can be ascribed to the formationan insulating layer on the film surface and the voltage dracross this preventing any further electrochemical proces

However the insulating layer produced in this way is tthin for device application. In order to obtain thicker oxdized layers, we increase the voltage, being careful that afilm–solution interface~where the reaction occurs! the dif-ference in potential does not exceed the decompositiontential ~;1.6 V!. The procedure can be repeated sevetimes in order to reach the desired thickness.

The optimized procedure consists of increasing the vage stepwise approximately 1.5 V, three or four times astopping the process when the resistance of the chareaches a value corresponding to a few nanometers tconducting layer~Fig. 6!. The I –V characteristics of such abarrier have strong nonlinear behavior.

No structural or morphological modifications are oserved on samples prepared in such a way. In Figs. 7 anthe XRD pattern and the AFM images of an as-grown film‘‘low and stepwise increased voltage’’ oxidized film, andhigh ~15 V! voltage oxidized film are shown, respectivelThe sample in Fig. 7~b! shows structural and morphologicaproperties identical to the as-grown film and the finite sfringes are unmodified by the electrochemical treatmentshown in the inset of Fig. 7.

As confirmation of decomposition of the STO phawhen the electrochemical reaction is carried out at higvoltage ~or current!, the XRD pattern in Fig. 7~c! shows asignificant reduction of STO reflection intensity, also the s

FIG. 5. Cyclic voltammetry of the oxidation process.

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face morphology is dramatically affected by the process@Fig.8~c!#. No additional Bragg peaks are observed, suggesthat the decomposition products do not have any long racrystalline structures. The latter kind of sample@Fig. 8~c!# ismorphologically very similar to the regions modified bybiased AFM tip in Ref. 35, suggesting that possibly the sachemical transformations are involved.

B. Electronic properties of films and devices

With respect to the dielectric properties of oxidized briers, we measure the capacitance via impedance versusquency measurements on large area~;2 mm2! devices, as-suming a simpleRC-series model. A value of 0.2mF isobtained, corresponding, in parallel plate geometry, tovalue of« r;300 of the dielectric constant of the barrier,agreement with the literature on dielectric propertiesSrTiO3 .3

A typical I –V curve of a 30 nm barrier and the sourcedrain channel resistance (RSD) variation as a function of applied gate voltage are shown in Figs. 9~a! and 9~b!, respec-tively. The I –V characteristic of the gate contact is strongnonlinear and shows significant asymmetry in the positand negative branches that is ascribed to the Schottky eintrinsic to the junction involved~Ag/STO!. The leakage cur-rent is probably related to defect conduction through the brier.

FIG. 6. Schematic of the oxidation procedure. From bottom to top: Apppotential increased stepwise; resulting anodization current; and monitchannel resistance.

FIG. 7. X-ray diffraction patterns of a film after~b! low voltage ~,4 V!oxidation and a film after~c! high voltage ~15 V! oxidation. The as-deposited SrTiO3 thin film in ~a! is shown for comparison. In the insemagnification of the~b! pattern around 002 reflection showing the finite sieffect.

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The application of the electric field induces significavariation in the channel conductivity. As expected forn-type semiconductor the application of positive bias togate induces an accumulation of negative charge carrierthe channel, thus reducing the channel resistivity asserved. Negative bias of the gate electrodes, on the ohand, depletes the interface layer of negative charge carand decreases the conductivity. Significant channel condtivity variation is observed in the region between22 and 0.5V where no leakage current through the barrier is detecOutside this voltage range the influence of the leakagerent on the channel becomes visible. Confirmation thatleakage current does not affect the measurements co

ded

FIG. 8. AFM images of the surface of~a! an as-deposited SrTiO3 thin filmand ~b! low voltage~,4! and ~c! high voltage~15 V! oxidized films. Theroot mean square roughness calculated for the three images is 0.7, 1.1110 Å, respectively.

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from the fact that curves measured at different applsource–drain currents nearly overlap.

Complete voltage sweeping cycles are shown in Fig.where the relative resistance modulation versus the gateage applied is plotted for two different temperatures. ARSD

modulation capability of;55% and;70% is reached at 300and 275 K, respectively. The increase in channel resistamodulation by cooling the sample is easy explained ifsemiconducting behavior of the channel resistance andtemperature dependence of the dielectric permittivity ofinsulating barrier are taken into account. Further decreathe temperature results in damaging the device due todifferent thermal coefficient expansion of the epoxy reused.

The curves in Fig. 10 show clear hysteretic behavior;origin of this hysteresis is still under investigation, but itprobably related to electromigration and ordering of oxygvacancies36 which induce slow changes in the dielectric co

FIG. 9. ~a! Gate barrier current characteristic and~b! source–drain resis-tance plotted as a function of the gate voltage applied. Channel resismodulation of about 60%, independent of the current fed into the cha~40 and 400 nA!, is observed in the22.040.5 V voltage range.

FIG. 10. Source–drain resistance vs the gate voltage for complete swecycles at 300 and 275 K.

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stant. A space charge effect due to trapped charges thattribute in different ways to the field effect is also plausibl

In order to confirm electrostatic modulation of thcharge carrier density in the device channel, we performmultaneous resistivity and Hall effect measurements durgate biasing. The devices typically used for transport prerties measurements have a smaller area (A51.531022 mm2), a channel~effective! thickness of about 5 nmand barrier thickness of 40 nm. With this device geomewe calculate the channel resistivity, the Hall coefficient, athus carrier density and the Hall mobility defined asRH /r.The device capacitance is obtained by using dielectric pmittivity of 300 and assuming planar plate geometry. TDebye length is estimated simply using the hypothesis ofhomogeneous semiconducting channel as

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whereKB is the Boltzmann constant,n is the evaluated carried density, and«5300«0 . The results obtained at zero gavoltage are summarized in Table I. The Debye length, beof the order of the effective thickness of the conducting laof the device, accounts for the large resistance variationserved in our samples.

The evolution of the Hall voltage as a function of thmagnetic field for different gate voltages is plotted in Fig. 1A clear effect on the slope, related to the change in cardensity induced by the field effect, is present. The corsponding number of carriers evaluated by consideringthe field effect acts on the entire film ranges from 1

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TABLE I. Main device parameters estimated at zero gate voltage. Theelectric permittivity is calculated by measuring the capacitance of an angous device with a larger area (A52 mm2; C50.2mF).

Channel thickness dc 5 nmBarrier thickness db 40 nmDielectric permittivity « r ;300Resistivity r 0.08V cmHall coefficient RH 0.31 cm3/CCarrier n 2.0231019 cm23

Hall mobility m 3.9 cm2/V sDebye length lD 4.6 nmCapacitance C 1.50 nF

FIG. 11. Hall voltage as a function of the magnetic field for different gabiases.

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31019 to 2.631019 cm23 and it is shown in Fig. 12, plottedalong with the number of carriers estimated from the retivity assuming the Hall mobility to be constant and indepedent of the gate voltage. In the same plot, we show therier density variation for each gate biasing voltagcalculated from the device capacitance. As one can seethree independent estimates of charge density modulationin good agreement. These results confirm the electrosnature of conductivity changes due to carrier modulationthe field effect.

IV. CONCLUSION

Summarizing, we are able to restore the stoichiomeoxygen content in oxygen deficient STO thin films by anooxidation. Control of the penetration depth of the electchemical process allows us to produce insulating layersdesired thickness on the surface of conducting layers.insulating properties of such anodic oxidized barriersgood enough to realize field effect devices fully basedstrontium titanate, in which resistance modulation up to 7can be observed. These results are in agreement withgeometrical size of our devices and with the carrier denof the underlying conducting oxygen deficient STO thin film

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FIG. 12. Number of charge carriers in the channel as a measurementstion of gate biasing obtained from the Hall effect and from conductivity. Tcontinuous line represents the charge induced calculated from the dcapacitance.

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